1. Field of the Invention
The invention relates to method and apparatus for controlling initiation of homogeneous-charge, compression-ignition (HCCI) over a wide range of load in diesel-cycle engines to reduce NOx nd PM emissions. The field of application is internal combustion engines for motor vehicles.
2. The Prior Art
The growing use of diesel-cycle engines in motor vehicles greatly adds to the atmospheric presence of pollutants such as oxides of nitrogen and particulate matter. Conventional diesel-cycle engines emit nitrogen oxide (NOx) and particulate matter (PM) substantially in excess of levels achievable in Otto-cycle (e.g., gasoline homogeneous-charge) engines, yet diesel-cycle engines achieve substantially better fuel economy. Because of the higher fuel economy, diesel-cycle engines dominate the heavy-duty truck market and much of the off-road commercial vehicle market, with growing penetration in light duty trucks. Thus, technology which could substantially reduce NOx and PM emissions in a cost effective and efficient manner from diesel-cycle engines is highly desired.
A substantial body of prior art describes the operation of homogenous-charge, compression-ignition (HCCI) engines. A homogenous-charge of fuel and air (oxygen) will auto-ignite during compression at a particular compression level (e.g., compression ratio), depending primarily on (1) the nature of the fuel (e.g., octane level), (2) concentration of the reactants (i.e., fuel and oxygen), and (3) the initial temperature of the charge mixture of fuel and air (including any recirculated exhaust gas). The compression of the charge both increases temperature and concentration of reactants, as well as increases pressure. There is one compression ratio for a given set of starting conditions where auto-ignition (i.e., compression ignition) will occur. If that compression ratio is the same as the mechanical compression ratio of the engine, then combustion will occur at the xe2x80x9crightxe2x80x9d time, i.e., with combustion initiation at piston top dead center (TDC) and peak cylinder pressure occurring generally within 10 degrees of crank angle (depending on engine speed), and emissions are low and efficiency is high. However, if the auto-ignition compression ratio is lower than the compression ratio of the engine, then combustion will occur before piston top dead center (TDC) and the engine will knock unacceptably. If the auto-ignition compression ratio is higher than the compression ratio of the engine, then misfire will occur and the engine will not operate. Therefore, the primary problem with the prior art (and the commercialization limitation of this combustion process) is the absence of an acceptable means of controlling the initiation of HCCI over the range of operating conditions (e.g., ambient temperature and load) necessary for a practical engine.
The present invention achieves control of the initiation of HCCI by mechanically controlling the engine compression ratio during engine operation so that for a particular set of operating conditions the initiation of HCCI will occur at an optimum condition when the piston has reached near TDC, generally within five crank angle degrees before to ten crank angle degrees after TDC depending on engine speed. One method of controlling engine compression ratio is to change the stroke of the piston by means such as; (1) raising or lowering the centerline of the crankshaft, (2) changing the effective length of the piston-to-crankshaft connecting rod, or (3) changing the effective length of the piston (and thus its displaced volume) above the piston/rod attachment. Another method of controlling engine compression ratio is to vary the height of the engine head above the TDC position of the piston.
Unfortunately, changing the compression ratio by any of these means also changes the expansion ratio of the engine and thus its thermal efficiency. For example, lowering the compression ratio of an engine to, for example 6, to avoid HCCI before piston TDC and to thereby avoid knock, would do so at the expense of engine efficiency. Also, such means of changing the compression ratio retains maximum compression at piston TDC. Given the very rapid combustion associated with HCCI, the peak combustion pressure generally occurs within ten crank angle degrees of TDC and for high engine loads produces undesirably high peak cylinder pressure and associated increased noise.
Accordingly, the present invention provides a diesel-cycle engine (capable of operating on a variety of fuels including gasoline and diesel) including a plurality of combustion cylinders and a first piston reciprocably mounted within each of the combustion cylinders. The piston presents a first face defining one boundary of a combustion chamber within a combustion cylinder and a head covers the combustion cylinders with a plurality of cylindrical recesses, each cylindrical recess opening into a respective one of the combustion cylinders. A second piston is reciprocably mounted in each of the cylindrical recesses in the head and presents a second piston face defining a second boundary of the combustion chamber. A fuel-air mixture is formed in a conventional manner to strive for a homogeneous mixture, with fuel injected into the air charge earlier than in a conventional diesel engine. A fuel-air mixture can be introduced into each cylinder, in succession, through a selected one of plural intake ports formed in the head, as practiced in conventional gasoline engines, or the fuel may be added to the air charge during air intake or compression.
A controller is provided for moving the second piston from a retracted position outward in the cylindrical recess in the head, to an extended position, during the end of each compression stroke (generally within five crank angle degrees before piston TDC) or the beginning of the expansion stroke (generally within ten crank angle degrees after piston TDC) of said piston, to reduce the volume of the combustion chamber and increase the compression ratio to a level causing auto-ignition of the fuel-air mixture.
In a preferred embodiment, the engine is further provided with a sensor for determining power demanded of the engine and with a controller for controlling the extended position of the second piston and thereby varying the compression ratio in accordance with the sensed power demand.
In accordance with another preferred feature of the present invention, the first piston has a face defining one boundary of the combustion chamber and a cylindrical recess formed therein axially aligned with a cylindrical recess in the engine head and, preferably, of the same diameter as the cylindrical recess in the engine head.
In accordance with another preferred aspect of the present invention, the second piston is a free-floating double face piston having one face defining the second boundary of the combustion chamber and a third face defining a control chamber in cooperation with a cylindrical recess in the engine head. An inlet port and an outlet port are provided for introducing hydraulic fluid to and exhausting hydraulic fluid from the control chamber. Each of these ports connects to a line having an on/off control valve therein whereby the second piston can be moved to its extended position by introduction of high pressure fluid into the control chamber and in another embodiment can be returned to its retracted position by the force of the expanding combustion gases in a power stroke.
In another aspect, the present invention provides a method for operation of the above-described engine, the method including moving the second piston outward from its retracted position to an extended position within a cylindrical recess within the head, to initiate each combustion stroke of the first piston, after the first piston has reached a point near top dead center, to reduce the volume of the combustion chamber and to increase the compression ratio to a level causing auto-ignition of a fuel-air mixture within the combustion chamber. The method of the present invention preferably includes the sensing of power demanded of the engine, e.g., by depression of an accelerator pedal, and controlling the extended position of the second piston, and thereby controlling the compression ratio, in accordance with the sensed power demand.
The preferred embodiment of the present invention maintains a high expansion ratio to maintain high efficiency by providing a method of operation and a means for final charge compression when the piston has already reached near TDC. This avoids engine knocking while providing sufficient compression to auto-ignite low fuel concentrations (light load) under even low charge temperatures.
For example, a fuel that would auto-ignite at a compression ratio of 6 under conditions of high load (maximum fuel concentration) and maximum expected initial charge temperature, would not auto-ignite at lower loads or temperatures and would thus need a means to increase compression ratio under those conditions. In this example, a preferred embodiment of the present invention would provide a conventional piston and crankshaft mechanism with a compression ratio of 6 in the conventional manner, but would also provide a movable surface for the combustion chamber, for example a second piston mounted in the head, which would be able to rapidly further reduce the volume of the combustion chamber (and thus increase compression ratio) after the piston has reached near TDC. The present invention thus provides a method of operation and a means for controlling HCCI at or near piston TDC while maintaining high compression and expansion ratios necessary to maintain high engine efficiency over a range of operating conditions.